Process for fabricating layered superlattice materials and making electronic devices including same

ABSTRACT

A liquid precursor containing a metal is applied to a substrate, RTP baked, and annealed to form a layered superlattice material. Special polyoxyalkylated precursor solutions are designed to optimize polarizability of the corresponding metal oxide materials by adding dopants including stoichiometric excess amounts of bismuth and tantalum. The RTP baking process is especially beneficial in optimizing the polarizability of the resultant metal oxide.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/065,656 filed May 21, 1993 (now U.S. Pat. No. 5,434,102),which is a continuation-in-part of U.S. patent application Ser. Nos.07/981,133 filed Nov. 24, 1992 (now U.S. Pat. No. 5,423,285) and07/965,190 filed Oct. 23, 1992 which in turn are continuations-in-partof U.S. patent application Ser. No. 07/807,439 filed Dec. 13, 1991,which is a continuation-in-part of U.S. patent application Ser. No.07/660,428 filed Feb. 25, 1991 (all now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention in general relates to the fabrication of layeredsuperlattice materials, and more particularly to fabrication processesthat provide low fatigue ferroelectric and reliable high dielectricconstant integrated circuit devices that are unusually resistant todegradation.

2. Statement of the Problem

Bulk ferroelectric layered perovskite-like superlattice materials areknown as phenomenonological curiosities. These materials comprise abroad class of ferroelectrics, and were reported by G. A. Smolenskii, V.A. Isupov, and A. I. Agranovskaya in Chapter 15 of the book,Ferroelectrics and Related Materials, ISSN 0275-9608, [V.3 of the seriesFerroelectrics and Related Phenomena, 1984] edited by G. A. Smolenskii.As is understood in the art, these materials typically include an oxygenoctahedral positioned within a cube formed of A-site metals at the cubecorners with the oxygen atoms occupying the planar face-centers of thecube and a B-site metal occupying the center of the cube. Thesematerials have previously not been considered to be useful as thin-filmcomponents for integrated circuits due to rapid polarization fatigue inthin-films and the high temperatures that are required to anneal amixture of powdered superlattice-forming metals into an orderedsuperlattice.

Rapid thermal processing and furnace annealing in an atmosphere ofoxygen are several of many processes that are well-known in thethin-film fabrication technology, See for example, "Process Optimizationand Characterization of Device Worthy Sol-Gel Based PZT forFerroelectric Memories", B. M. Melnick, J. D. Cuchiaro, L. D. McMillan,C. A. Paz De Araujo, and J. F. Scott in Ferroelectrics, Vol 109, pp.1-23 (1990). It is also known to add excess lead in fabricating PZTusing a spin-on and annealing process to account for lead lost as leadoxide vapor in the fabrication process. See U.S. Pat. No. 5,028,455issued to William D. Miller et al. It is also known to add excess Bi₂ O₃when fabricating a bismuth titanate thin film using sputtering tocompensate for the loss of this component in the sputtering process. See"A New Ferroelectric Memory Device, Metal-Ferroelectric-SemiconductorTransistor", by Shu-Yau Wu, IEEE Transactions On Electron Devices,August 1974, pp. 499-504. E. C. Subbarao, in "A Family of FerroelectricBismuth Compounds", J. Phys. Chem. Solids, V. 23, pp. 665-676 (1962),discloses the creation of solid solutions of some layered superlatticematerials and that several of their physical parameters, i.e., thedielectric constant and Curie temperature change as the proportions ofthe various elements comprising the solid solution change.

Hundreds of processes and parameters can potentially can affect thequality of a ferroelectric material. Those skilled in the art have beensearching for more than thirty years to develop ferroelectric materialswith properties of extremely low fatigue rates and polarizabilities ashigh as 25.

SOLUTION TO THE PROBLEM

The present invention solves the above problem by providing a method offabricating a layered superlattice material having exceptionally highpolarizability and low fatigue, as well as layered superlatticematerials produced according to this process.

Copending U.S. patent application Ser. No. 07/981,133, describes amethod of fabricating layered superlattice thin films that results inelectronic properties for these materials several times better than thebest previously known. Specifically, the Ser. No. 07/981,133 applicationteaches that the Smolenskii-type layered superlattice materials are farbetter suited for ferroelectric and high dielectric constant integratedcircuit applications than any prior materials used for theseapplications. These layered superlattice materials comprise complexoxides of metals, such as strontium, calcium, barium, bismuth, cadmium,lead, titanium, tantalum, hafnium, tungsten, niobium zirconium, bismuth,scandium, yttrium, lanthanum, antimony, chromium, and thallium thatspontaneously form layered superlattices, i.e. crystalline lattices thatinclude alternating layers of distinctly different sublattices, such asa ferroelectric and non-ferroelectric sublattices. Generally, eachlayered superlattice material will include two or more of the abovemetals; for example, strontium, bismuth and tantalum form the layeredsuperlattice material strontium bismuth tantalate, SrBi₂ Ta₂ O₉. Thisdisclosure provides improvements in the fabrication process thattogether approximately double the values of the critical ferroelectricparameters, such as the polarizability, over the values obtained withthe basic process described in the copending Ser. No. 07/981,133application.

Broadly speaking, the present invention pertains to materials andmethods for fabricating a layered superlattice material. The methodincludes the steps of providing a substrate and preparing a liquidprecursor including a mixture of polyoxyalkylated metal moieties ineffective amounts for optimizing an electrical property in layeredsuperlattice materials to be produced from the precursor. The precursorincludes a strontium metal moiety, a bismuth metal moiety, and at leastone additional metal moiety selected from a group consisting oftantalum, niobium, titanium, zirconium, and mixtures thereof. The methodfurther includes the steps of applying said precursor to the substrateand heating the precursor on the substrate to form a layeredsuperlattice material containing the mixture of metal moieties.

Prebaking the substrate to a temperature above the temperature of theheating step prior to the step of applying the precursor to thesubstrate is often a key factor in obtaining high performanceferroelectrics.

The mixture of metal moieties in the precursor solution precursorsolution preferably reflects a stoichiometric mixture of metals definedby an empirical formula:

    Sr.sub.w [Bi.sub.4-2x+α {(Ta.sub.y,Nb.sub.1-y).sub.x,(Ti.sub.z,Zr.sub.1-z).sub.2-2x }.sub.2 O.sub.15-6x ].sub.c,                                      (1)

wherein w, c, x, y, z, and α represent numbers defining stoichiometricportions of formula atoms, 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0, (x-2)≦α≦1.6(2-x),0≦w≦1.0, c≧1.0, and (c÷w)≧1. In one preferred embodiment, w≦1, and c=1.In another preferred embodiment, w=1 and c≧1. In yet another preferredembodiment, 0.6≦w≦1.0 and c≧1. These preferred embodiments all reflectthe addition of excess bismuth and B-site metals with respect to theSmolenskii formula

    A.sub.m-1 Bi.sub.2 B.sub.m O.sub.3m+3,                     (2)

wherein A is an A-site metal, B is a B-site metal, O is oxygen, and m isa number equal to at least one.

The departures from the Smolenskii formula are derived from addingdopants to the precursor solutions. Preferred dopants include bismuthand/or tantalum in amounts up to at least about 20 mol % of theempirical formula. An excess bismuth portion ranging from 5% to 10% ofthe stoichiometrically required amount can be added to the precursorsolution for purposes of compensating for anticipated bismuthvolatilization losses during the annealing process. In the case of rapidthermal processing, where losses are lower, the dopants can preventbismuth-induced shorting of the metal oxide materials, thereby expandingthe range of A-site metals that may be employed for purposes ofincreasing polarizability.

An especially preferred embodiment of the present invention includespreparing the precursor solution to reflect a stoichiometric balance ofmetal moieties defined by the empirical formula

    Sr.sub.w (Bi.sub.α Ta.sub.2).sub.y O.sub.z,          (3)

wherein w, y, z, and α represent numbers corresponding to stoichiometricportions of formula atoms, 0≦w≦1.0, 0≦y≦1.0, 2≦α≦2.2, 0.6≦(w÷y)≦1.0, andz is sufficient to balance the formula charge by providing sufficientoxygen anion to balance the Sr, Bi, and Ta cations. Yet anotherespecially preferred empirical formula is

    (SrBi.sub.α).sub.w Ta.sub.y O.sub.z,                 (4)

wherein w, y, and z represent numbers corresponding to stoichiometricportions of formula substituents, 0≦w≦1.0, 0≦y≦1.0, 2≦α≦2.2, 2≦y/w, andz is sufficient to balance the charge formula charge by providingsufficient oxygen anion to balance the Sr, Bi, and Ta cations.

The heating or annealing step preferably includes a step of rapidthermal processing the precursor in an oxygen atmosphere to a targetmaximum temperature ranging from about 500° C. and about 850° C., andmore preferably ranging from 675° C. to 825° C. Even more preferably,the heating step includes ramping a temperature of the precursor at arate between 1° C. per second and 200° C. per second, and holding theprecursor film at a target temperature and holding it at the targettemperature for from 5 seconds to 300 seconds. A step of furnaceannealing the substrate may be conducted subsequent to the rapid thermalprocessing step. The furnace anneal is most preferably conducted at atemperature ranging from about 700° C. to about 850° C.

The method can include additional steps, such as forming a ferroelectriccapacitor including the layered superlattice material between twoelectrodes, patterning the device, and subsequently performing a furtheranneal.

The precursor can be manufactured with selected ingredients foroptimizing the polarizability of a given type of layered superlatticematerial, e.g., strontium bismuth tantalate materials. The first step ofthe optimization method includes the steps of selecting a primaryprecursor liquid including a combination of metal moieties in effectiveamounts for yielding a layered superlattice material having a firstempirical formula providing a first level of electronic performance. Themetal moieties at least include a bismuth moiety, an A-site metalmoiety, and a B-site metal moiety. A series of secondary precursorliquids are formulated by varying a relative proportion of at least oneof the metal moieties with respect to the first empirical formula. Aplurality of representative metal oxide materials are produced from eachof the primary and secondary precursor liquids. An electrical propertyis measured in each of the representative metal oxide materials. Anoptimal content of precursor ingredients is identified through acomparison of the electrical measurements. The electronic properties mayalso include one or more properties selected from the group consistingof polarizability, coercive field, leakage current, dielectric constant,and fatigue. The elements that form the solid solution may includeelements selected from the group comprising tantalum, niobium, titanium,zirconium and many other elements. Electronic performance can also beoptimized by conducting a sensitivity analysis based upon the effects ofprocess parameters such as anneal temperature, anneal time, thermalramping rates, drying of the precursor prior to the anneal, andprebaking of the substrate.

The preferred process results in a layered superlattice material havingan average grain size of from 20 to 200 nm. As compared to the priorart, the process reduces the grain size of the material while reducingthe distribution of grain sizes, thus improving the crystallinity of thefilm. At the same time the process shortens the fabrication time, sincethe material reaches high values of 2Pr with shorter furnace annealtime.

The methods described above result in layered superlattice materialshaving excellent electronic properties. For example, ferroelectriclayered superlattice materials with polarizabilities, 2Pr, higher than25 microcoulombs per square centimeter have been fabricated. Numerousother features, objects and advantages of the invention will becomeapparent from the following description when read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the preferred embodiment of a process forpreparing a thin film of a layered superlattice material according tothe invention;

FIG. 2 is a top view of a wafer on which thin film capacitors fabricatedby the process according to the invention are shown greatly enlarged;

FIG. 3 is a portion of a cross-section of FIG. 2 taken through the lines3--3, illustrating a thin film capacitor device fabricated by theprocess according to the invention;

FIG. 4 is a cross-sectional illustration of a portion of an integratedcircuit fabricated utilizing the process of, the invention;

FIG. 5 shows hysteresis curves at ±2, ±4, ±6 and ±8 volts for a sampleof SrBi₂ Ta₂ O₉ having 10% excess bismuth and fabricated according tothe invention;

FIG. 6 is a graph of 2Pr versus number of cycles for the sample of FIG.5 illustrating the excellent resistance to fatigue of the materialaccording to the invention;

FIG. 7 shows the hysteresis curves at 2, 4, and 6 volts for samples ofstrontium bismuth tantalate fabricated utilizing precursor solutionshaving different bismuth content;

FIG. 8 shows graphs of 2Pr and 2Ec for the 6 volt hysteresis curves ofFIG. 10;

FIG. 9 shows graphs of 2Pr versus number of cycles for the samples ofFIG. 10;

FIG. 10 shows graphs of 2Pr and 2Ec versus first anneal temperature forsamples of strontium bismuth tantalate;

FIG. 11 shows hysteresis curves at 2, 4, and 6 volts for samplescomprising different solid solutions of strontium bismuth tantalate andstrontium bismuth niobate;

FIG. 11 shows graphs of 2Pr and 2Ec for samples comprising differentsolid solutions of strontium bismuth tantalate and strontium bismuthniobate;

FIG. 12 shows graphs of 2Pr versus number of cycles for samplescomprising different solid solutions of strontium bismuth tantalate andstrontium bismuth niobate;

FIG. 13 shows hysteresis curves at 2, 4, 6, and 8 volts for samplescomprising different solid solutions of strontium bismuth titanate andstrontium bismuth tantalate;

FIG. 14 shows hysteresis curves at 2, 4, and 6 volts for samplescomprising different solid solutions of strontium bismuth titanate andstrontium bismuth niobate;

FIG. 15 shows a triangular diagram showing 2Pr for a variety ofdifferent solid solutions of strontium bismuth titanate, strontiumbismuth tantalate, and strontium bismuth niobate;

FIG. 16 shows graphs of 2Pr versus percentage of titanium for solidsolutions of strontium bismuth titanate, strontium bismuth tantalate,and strontium bismuth niobate for differing relative amounts of tantalumand niobium;

FIG. 17 is a table listing 2Pr and 2Ec for various solid solutions ofstrontium bismuth titanate, strontium bismuth tantalate, and strontiumbismuth niobate;

FIG. 18 shows graphs of 2Pr versus number of cycles for some of thesamples of FIGS. 13 through 15;

FIG. 19 shows hysteresis curves at 2, 4, and 6 volts for samples withdifferent solid solutions of strontium bismuth titanate and strontiumbismuth zirconate;

FIG. 20 shows graphs of 2Pr and 2Ec versus zirconium percentage for thesamples of FIG. 19;

FIG. 21 shows graphs of 2Pr versus number of cycles for two differentsamples of solid solutions of strontium bismuth titanate and strontiumbismuth zirconate having different percentages of zirconium;

FIG. 22 depicts 2Pr versus Ec for hysteresis measurements from numerouscapacitors manufactured according to different dielectric materialformulations having various Sr concentrations in stoichiometricproportion with other formula elements;

FIG. 23 depicts hysteresis curves for capacitors used to construct theFIG. 2 plot having a dielectric material including a Sr_(w) Bi₂ Ta₂O_(8+w) formulation, with these curves representing initial polarizationmeasurements at the respective voltages;

FIG. 24 depicts hysteresis curves for capacitors used to construct theFIG. 22 plot, with these curves representing second measurementsconducted on the FIG. 23 samples at the respective voltages;

FIG. 25 depicts hysteresis curves for capacitors used to construct theFIG. 37 plot having a dielectric material including a Sr_(w) Bi₂.2 Ta₂O₈.3+w (ten mol % excess Bi) formulation;

FIG. 26 depicts a plurality of hysteresis curves taken from differentsurface portions of a single capacitor or wafer having the formulationSr₀.8 Bi₂ Ta₂ O₈.8 ;

FIG. 27 depicts hysteresis curves similar to those of FIG. 26, butrepresenting measurements taken from a dielectric material having aformulation Sr₀.9 Bi₂ Ta₂ O₈.9 ;

FIG. 28 depicts hysteresis curves similar to those of FIG. 26, butrepresenting measurements taken from a dielectric material having aformulation Sr₁ Bi₂ Ta₂ O₉ ;

FIG. 29 depicts hysteresis curves similar to those of FIG. 26, butrepresenting measurements taken from a dielectric material having aformulation Sr₁.1 Bi₂ Ta₂ O₉.1 ;

FIG. 30 depicts hysteresis curves similar to those of FIG. 26, butrepresenting measurements taken from a dielectric material having aformulation Sr₁.2 Bi₂ Ta₂ O₉.2, wherein the Sr-rich formation has causeda short in the capacitor circuit;

FIG. 31 depicts hysteresis curves similar to those of FIG. 26, butrepresenting measurements taken from a dielectric material having aformulation Sr₁.4 Bi₂ Ta₂ O₉.4, wherein the Sr-rich formation has causeda short in the capacitor circuit;

FIG. 32 depicts results for fatigue measurements conducted on dielectricmaterial samples having strontium molar portions ranging from 0.5 to1.1;

FIG. 33 depicts several hysteresis curves taken from a capacitor havinga dielectric curve of the formulation SrBi₂.2 Ta₂ O₉.3, wherein thecapacitor exhibits shorting or a near total failure of the dielectriclayer;

FIG. 34 depicts several hysteresis curves taken from a capacitor havinga dielectric material formulation SrBi₂.2 Ta₂ O₉.3, like the formulationof FIG. 33, but with a ten mole percent dopant of Bi₂ Ta₂ O₈ added,wherein the dielectric layer has not failed;

FIG. 35 depicts a hysteresis curve for a dielectric material having aformulation including SrBi₂ Ta₂ O₉ but with 10 mole percent Bi₂ Ta₂ O₈added as a dopant, wherein the dielectric layer has not failed;

FIG. 36 depicts a hysteresis curve for a dielectric material having aformulation including SrBi₂ Ta₂ O₉ but with 20 mole percent TaO added asa dopant, wherein the dielectric layer has not failed; and

FIG. 37 depicts hysteresis curves for capacitors having respectivedielectric materials including the base formulation SrBi₂.2 Ta₂ O₉.3with various dopants added, including BiO, TaO, and Bi₂ Ta₂ O₈.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Overview

Directing attention to FIGS. 2 and 3, a wafer 10 containing numerouscapacitors 12, 14, 16 etc. is shown. FIG. 2 is a top view of the wafer10 on which the thin film capacitors 12, 14, 16 etc. fabricated by theprocess according to the invention are shown greatly enlarged. FIG. 3 isa portion of a cross-section of FIG. 2 taken through the lines 3--3bisecting capacitor 16. Referring to FIG. 3, the wafer 10 includes asilicon substrate 22, a silicon dioxide insulating layer 24, a thinlayer of titanium 26 which assists adhesion of the next layer, which isa platinum electrode 28, in adhering to the silicon dioxide 24, a layerof layered superlattice material 30, and another platinum electrode 32.After the layers 24, 26, 28, 30, and 32, are deposited, the wafer isetched down to layer 28 to form the individual capacitors 12, 14, 16,etc. which are interconnected by the bottom electrode 28.

The present invention primarily involves the method of creating thelayer 30 of layered superlattice material. As mentioned above, theselayered superlattice materials comprise complex oxides of metals, suchas strontium, calcium, barium, bismuth, cadmium, lead, titanium,tantalum, hafnium, tungsten, niobium zirconium, bismuth, scandium,yttrium, lanthanum, antimony, chromium, and thallium that spontaneouslyform layered superlattices, i.e. crystalline lattices that includealternating layers of distinctly different sublattices. Generally eachlayered superlattice material will include two or more of the abovemetals; for example, barium, bismuth and niobium form the layeredsuperlattice material barium bismuth niobate, BaBi₂ Nb₂ O₉. The material30 may be a dielectric, a ferroelectric, or both. If it is a dielectric,the capacitor 16 is a dielectric capacitor, and if the material 30 is aferroelectric, then capacitor 16 is a ferroelectric capacitor

The layered superlattice materials may be summarized more generallyunder the formula:

    A1.sub.w1.sup.+a1 A2.sub.w2.sup.+a2 . . . A.sub.j.sub.wj.sup.+aj S1.sub.z1.sup.+s1 S2.sub.x2.sup.+s2 . . . Sk.sub.xk.sup.+sk B1.sub.y1.sup.+b1 B2.sub.y2.sup.+b2 . . . Bl.sub.yl.sup.+bl Q.sub.z.sup.-2,                                           (5)

where A1, A2 . . . Aj represent A-site elements in the perovskite-likestructure, which may be elements such as strontium, calcium, barium,bismuth, lead, and others S1, S2 . . . Sk represent super-latticegenerator elements, which usually is bismuth, but can also be materialssuch as yttrium, scandium, lanthanum, antimony, chromium, thallium, andother elements with a valence of +3, B1, B2 . . . Bl represent B-siteelements in the perovskite-like structure, which may be elements such astitanium, tantalum, hafnium, tungsten, niobium, zirconium, and otherelements, and Q represents an anion, which generally is oxygen but mayalso be other elements, such as fluorine, chlorine and hybrids of theseelements, such as the oxyfluorides, the oxychlorides, etc. Thesuperscripts in Formula (5) indicate the valences of the respectiveelements, and the subscripts indicate the number of moles of thematerial in a mole of the compound, or in terms of the unit cell, thenumber of atoms of the element, on the average, in the unit cell. Thesubscripts can be integer or fractional. That is, Formula (5) includesthe cases where the unit cell may vary throughout the material, e.g. inSr₀.75 Ba₀.25 Bi₂ Ta₂ O₉, on the average, 75% of the time Sr is theA-site atom and 25% of the time Ba is the A-site atom. If there is onlyone A-site element in the compound then it is represented by the "A1"element and w2 . . . wj all equal zero. If there is only one B-siteelement in the compound, then it is represented by the "B1" element, andy2 . . . yl all equal zero, and similarly for the superlattice generatorelements. The usual case is that there is one A-site element, onesuperlattice generator element, and one or two B-site elements, althoughFormula (5) is written in the more general form since the invention isintended to include the cases where either of the sites and thesuperlattice generator can have multiple elements. The value of z isfound from the equation:

    (a1w1+a2W2 . . . +ajwj)+(z1x1+x2x2 . . . +skxk)+(b1y1+b2y2 . . . +bjyj)=2z. (2)

Formula (5) includes all three of the Smolenskii type compounds. Thelayered superlattice materials do not include every material that can befit into the Formula (5), but only those which spontaneously formthemselves into crystalline structures with distinct alternating layers.It should be noted that the x, y, and z symbols in the Formula (5)should not be confused with the x, y, and z, symbols used in theFormulas (6) and (7) below. The Formula (5) is a general formula forlayered superlattice materials, while the Formulae (6) and (7) areformulae for solid solutions of particular layered superlatticematerials.

It should also be understood that the term layered superlattice materialherein also includes doped layered superlattice materials. That is, anyof the material included in Formula (5) may be doped with a variety ofmaterials, such as silicon, germanium, uranium, zirconium, fin orhafnium. For example, strontium bismuth tantalate may be doped with avariety of elements as given by the formula:

    (Sr.sub.1-x M1.sub.x)Bi.sub.2 (Nb.sub.1-y M2.sub.y)O.sub.9 +αM3, (6)

where M1 may be Ca, Ba, Mg, or Pb, M2 may be Ta, Bi, or Sb, with x and ybeing a number between 0 and 1 and preferably 0≦x≦0.2, 0≦y≦0.2, M3 maybe Si, Ge, U, Zr, Sn, or Hf, and preferably 0≦α≦0.05. Materials includedin this formula are also included in the term layered superlatticematerials used herein.

Similarly, a relatively minor second component may be added to a layeredsuperlattice material and the resulting material will still be withinthe invention. For example, a small amount of an oxygen octahedralmaterial of the formula ABO₃ may be added to strontium bismuth tantalateas indicated by the formula:

    (1-x) SrBi.sub.2 Ta.sub.2 O.sub.9 +xABO.sub.3,             (7)

where A may be Bi, Sr, Ca, Mg, Pb, Y, Ba, Sn, and Ln; B may be Ti, Zr,Hf, Mn, Ni, Fe, and Co; and x is a number between 0 and 1, preferably,0≦x≦0.2.

Likewise the layered superlattice material may be modified by both aminor ABO₃ component and a dopant. For example, a material according tothe formula:

    (1-x) SrBi.sub.2 Ta.sub.9 O.sub.9 +XABO.sub.3,+αM',  (8)

where A may be Bi, Sb, Y and Ln; B may be Nb, Ta, and Bi; M' may be Si,Ge, U, Ti, Sn, and Zr; and x is a number between 0 and 1, preferably,0≦x≦0.2, is contemplated by the invention.

FIG. 4 shows an example of the integration of a layered superlatticecapacitor 72 into a DRAM memory cell to form an integrated circuit 70such as may be fabricated using the invention. The memory cell 50includes a silicon substrate 51, field oxide areas 54, and twoelectrically interconnected electrical devices, a transistor 71 and aferroelectric switching capacitor 72. Transistor 71 includes a gate 73,a source 74, and a drain 75. Capacitor 72 includes first electrode 58,ferroelectric layered superlattice material 60, and second electrode 77.Insulators, such as 56, separate the devices 71, 72, except where drain75 of transistor 71 is connected to first electrode 58 of capacitor 72.Electrical contacts, such as 47 and 78 make electrical connection to thedevices 71, 72 to other parts of the integrated circuit 70. A detailedexample of the complete fabrication process for an integrated circuitmemory cell as shown in FIG. 4 is given in U.S. patent application Ser.No. 07/919,186, which is incorporated herein by reference. It should beunderstood that FIGS. 2, 3, 4 depicting the capacitors 12, 14, 16 etc.and integrated circuit 70 are not meant to be actual cross-sectionalviews of any particular portion of an actual electronic device, but aremerely idealized representations which are employed to more clearly andfully depict the structure and process of the invention than wouldotherwise be possible.

This disclosure describes the fabrication and testing of numerouscapacitors 12, 14, 16 having layers 22, 24, 26, 28, 30, and 32 made ofthe materials above, disclosing a wide spectrum of variations of thefabrication process according to the invention and a variety ofdifferent layered superlattice materials 30. It then discloses how thisdata is used to design and manufacture electronic devices utilizinglayered superlattice materials. It should be understood, however, thatthe specific processes and electronic devices described are exemplary,i.e., the invention contemplates that the layers in FIGS. 3 and 4 may bemade of many other materials than those mentioned above and describedbelow, there are many other variations of the process of the inventionthan can be included in a document such at this, and the method andmaterials may be used in many other electronic devices other than thecapacitors, such as 12, 14, 16 etc. and the integrated circuit 70. Itshould also be noted that the word "substrate" is used in both aspecific and a general sense in this disclosure. In the specific senseit refers to the specific silicon layer 22, conventionally called asilicon substrate, on which the exemplary electronic devices describedare fabricated. In a general sense, it refers to any material, object,or layer on which another layer or layers are formed. In this sense, forexample, the layers 22, 24, 26, and 28 comprise a substrate 18 for thelayer 30 of layered superlattice material 30.

A term that is used frequently in this disclosure is "stoichiometry" or"stoichiometric". As used herein, the term stoichiometric generallyexpresses a relationship between the precursor solution and the finallayered superlattice film 30. A "stoichiometric precursor" is one inwhich the relative proportions of the various metals in the precursor isthe same as the proportion in a homogeneous specimen of the intendedfinal layered superlattice thin film 30. This proportion is the onespecified by the formula for the final thin film 30.

2. Detailed Description of the Fabrication Process

Turning now to a more detailed description of the invention, a flowchart of the preferred embodiment of a process according to theinvention for preparing a thin film of a layered superlattice material,such as 30 and 60, and a device, such as 10 and 70 incorporating thematerial 30 and 60, is shown in FIG. 1. We shall first review each stepof the preferred process briefly, and then discuss the individual stepsin more detail and provide examples of the process. The first step 80 ofthe process is the preparation of the precursor or precursors, P1, P2,P3, etc. In the preferred embodiment the precursors are liquids in whicha compound or compounds of the metals to comprise the layeredsuperlattice material 30 are dissolved. The precursors are then mixed instep 81, and the mixed precursors are distilled in step 82. Then followsa solvent control and/or concentration control step 83. Generally thisstep is taken over two stages which may be separated considerably intime. In the first stage the mixed precursor is dissolved in a suitablesolvent and concentrated so as to provide a long shelve life. Justbefore use, the solvent and concentration may be adjusted to optimizethe electronic device that results from the process.

In parallel with the solvent and concentration control step 83, thesubstrate 18 is prepared. If the substrate is a metallized substrate,such as the substrate 18, then the substrate is provided in step 85A byforming the layers 22, 24, 26, and 28 and is then prebaked in step 86A.If the substrate is a non-metallized substrate, such as a silicon orgallium arsenide single crystal, the substrate is provided in step 85Band dehydrated in step 86B. In step 87 the substrate is coated with theprecursor. In the examples discussed below, the coating was done by aspin-on process, though a process such as a misted deposition process asdescribed in U.S. patent application Ser. No. 07/993,380, which ishereby incorporated by reference, or dipping or other suitable coatingprocess may be used. The coated substrate is then dried in step 88, andthe baked in an RTP (rapid thermal processor) unit. If the desiredthickness of the layer 30 is not obtained, then the series of coat, dry,and RTP bake steps 87, 88, and 89 are repeated as many times as requiredto build up the desired thickness. The wafer 10 is then annealed in step92, the top or second electrode 32 is deposited in step 93 by sputteringor other suitable process, and the wafer 10 is then annealed, again instep 94. The capacitor 16 is then structured by ion milling, chemicaletching, or other suitable process in step 95. Then follows a thirdanneal in step 96. This completes the process if a capacitor device asin FIG. 2 is the desired end result, however in the case of anintegrated circuit as in FIG. 4, there follows completion steps 97 suchas contact metalization, capping, etc. As will be discussed furtherbelow, not all of the steps outlined above are necessary for everydevice: some steps are optional and others are used only for certainlayered superlattice materials.

The preferred precursors solutions and their preparation in step 80 arediscussed in detail in U.S. patent application Ser. No. 07/981,133 whichis hereby incorporated herein by reference. Generally a metal or a metalcompound is reacted with a carboxylic acid, such as 2-ethylhexanoicacid, to produce a metal hexanoate, which is dissolved in a suitablesolvent or solvents, such as xylenes. Other metal-organic acid esters inaddition to the 2-ethylhexanotates that may for suitable precursors whencompounded with a metal are the acetates and acetylacetonates. For somemetals, such as titanium, the precursor metal compound may comprise ametal alkoxide, such as titanium 2-methoxyethoxide. Other alkoxides thatmay be compounded with a metal and used as precursor compounds includethe methoxides, ethoxides, n-propoxide, iso-propoxides, n-butoxides,iso-butoxides, tert-butoxides, 2-methoxyethoxides, and2-ethoexyethoxides. The precursor metal compound is preferably dissolvedin a solvent having a boiling point greater than the boiling point ofwater, i.e. 100° C. A xylenes solvent works for most metals. For highlyelectropositive elements, the solvent preferably includes2-methoxyethanol or n-butyl acetate. Some solvents that may be used,together with their boiling points, include: alcohols, such as 1-butanol(117° C.), 1-pentanol (117° C.), 2-pentanol (119° C.), 1-hexanol (157°C.), 2-hexanol (136° C.), 3-hexanol (135° C.), 2-ethyl-1-butanol (146°C.), 2-methoxyethanol (124° C.), 2-ethoxyethanol (135° C.), and2-methyl-1-pentanol (148° C.); ketones, such as 2-hexanone (methyl butylketone) (127° C.), 4-methyl-2-pentanone (methyl isobutyl ketone) (118°C.), 3-heptanone (butyl ethyl ketone) (123° C.), and cyclohexanone (156°C.); esters, such as butyl acetate (127° C.), 2-methoxyethyl acetate(145° C.), and 2-ethoxyethyl acetate (156° C.); ethers, such as2-methoxyethyl ether (162° C.) and 2-ethoxyethyl ether (190° C.); andaromatic hydrocarbons, such as xylenes (138° C.-143° C.), toluene (111°C.) and ethylbenzene (136° C.).

The metal carboxylate may be further reacted with an alkoxide to form ametallic alkoxycarboxylate, which is particularly preferred for use inprecursor solutions. This preference exists because at least about 50%of the metal to oxygen bonds that will exist in the final superlatticematerial are believed to be formed in the precursor. Accordingly,crystalline materials formed from a liquid deposition process utilizingthese precursors exhibit superior electrical performance characteristicsdue to enhanced crystalline properties.

The precursors of the individual metals may be made separately and thenmixed, but generally they are all made together in the same containerand mixed as they are made. After mixing, the precursor solution may bedistilled to remove water and other undesirable impurities andby-products of the preparation process, although if the precursors andsolvents are available in pure enough states, the distillation step 81may be skipped. Distillation preferably occurs to a solution temperatureof at least 115° C., more preferably to at least 120° C., and mostpreferably to at least about 123° C. Distillation induces theelimination of reaction byproducts and drives various endothermicreactions to substantial completeness, e.g., the polyoxyalkylated metalportion preferably includes a fraction having the molecular formula:

    (R'--COO--).sub.a M(--O--R).sub.n,                         (9)

or

    (R'--C--O--).sub.a M(--O--M'(--O--C--R").sub.b-1).sub.n,   (10)

wherein M is a metal having an outer valence of (a+n) and M' is a metalhaving an outer valence of b, with M and M' preferably being selectedfrom the group consisting of tantalum, calcium, bismuth, lead, yttrium,scandium, lanthanum, antimony, chromium, thallium, hafnium, tungsten,niobium, zirconium, manganese, iron, cobalt, nickel, magnesium,molybdenum, strontium, barium, titanium, vanadium, and zinc; R and R'are respective alkyl groups preferably having from 4 to 9 carbon atomsand R" is an alkyl group preferably having from 3 to 8 carbon atoms. Thelatter formula, which has a central --O--M--O--M'--O-- structure, isparticularly preferred due to the formation in solution of at least 50%of the metal to oxygen bonds that will exist in the final solid metaloxide product. Similar --M--O--M'--O-- structures are obtainable fromreactions between metal alkoxycarboxylates and respective metal alkoxideor metal carboxylate reagents.

Exemplary chemical reactions that are useful in producing the aboveproducts include

    alkoxides--M.sup.+n +n R--OH→M(O--R).sub.n +n/2 H.sub.2 (11)

    carboxylates--M.sup.+n +n (R--COOH)→M(OOC--R).sub.n +n/2 H.sub.2 (12)

    alkoxycarboxylates--M(O--R').sub.n +b R--COOH+heat→(R'--O).sub.n-b M(OOC--R).sub.b +b HOR,                                   (13)

where M is a metal cation having a charge of n; b is a number of molesof carboxylic acid ranging from 0 to n; R' is preferably an alkyl grouphaving from 4 to 15 carbon atoms and R is preferably an alkyl grouphaving from 3 to 9 carbon atoms. According to a generalized reactiontheory, if a metal-alkoxide is added to the metal-alkoxycarboxylate, andthe solution is heated, the following reactions occur:

    (R--COO--).sub.x --M(--O--C--R').sub.a +M'(--O--C--R").sub.b →(R--COO--).sub.x --M--(O--M'(--O--C--R").sub.b-1).sub.a +a R'--O--C--R"                                              (14)

    (R--COO--).sub.x M(--O--C--R').sub.a +x M'(--O--C--R").sub.b →(R'--C--O--).sub.a --M(--O--M'(--O--C--R").sub.b-1).sub.x +x R--COO--C--R"                                             (15)

where M and M' are metals; R and R' are defined above; R" is an alkylgroup preferably having from about zero to sixteen carbons; and a, b,and x are integers denoting relative quantities of correspondingsubstituents corresponding to the respective valence states of M and M'.Generally the reaction of Equation (14) will predominate. Thus, ethershaving low boiling points are generally formed. These ethers boil out ofthe pre-precursor to leave a final product having a reduced organiccontent and the metal-oxygen-metal bonds of the final desired metaloxide already partially formed. If the heating is sufficient, some ofthe reaction (15) will also occur, creating metal-oxygen-metal bonds andesters. Esters generally have higher boiling points and remain insolution. These high boiling point organics slow down the drying processafter the final precursor is applied to a substrate, which tends toreduce cracking and defects; thus, in either case, metal-oxygen-metalbonds are formed and the final precursor performance is improved.

If a metal-carboxylate is added to the metal-alkoxycarboxylate and themixture is heated, the following reaction occurs:

    (R--COO).sub.x --M--(O--C--R').sub.a +x M'--(OOC--R").sub.b →(R'--C--O).sub.a --M--(O--M'--(OOC--R").sub.b-1).sub.x +x R--COOOC--R'                                              (16)

where R--COOOC--R' is an acid anhydride, and the terms are as definedabove. This reaction requires considerably more heat than do thereactions (14) and (15) above, and proceeds at a much slower rate. Thereaction products of equations (11)-(16) can be heated with excesscarboxylic acid to substitute carboxylate ligands for alkoxide ligands,thereby reducing the hydrolyzing ability of the carboxylated productsand increasing precursor shelf life.

The solvent type and concentration may then be adjusted in step 83either to prepare it for coating, if the coating is to be doneimmediately, or to provide a precursor with a long shelf life. If thesolvent control steps are such as to prepare a solution with a longshelf life, then just before coating, another adjustment will usually bedone to optimize the thin film. Some adjustments to produce a long shelflife and to produce high quality films are discussed in detail in U.S.patent application Ser. No. 07/981,133. It is a feature of the presentinvention that it has been found that while a single solvent precursor,such as the precursors with xylenes as the solvent described in theprior application, may have a long shelf life, adding a second solvent,or a plurality of solvents immediately prior to coating results in muchhigher quality thin films.

It has been found that a single solvent often cannot be found that hasthe optimum solubility, viscosity, and boiling point. The solubility ofthe metal compound in the solvent determines whether or not fineprecipitates occur; the viscosity determines the smoothness of thecoating process, and the boiling point determines how fast the solventvaporizes in the drying process, which effects whether defects appearduring the drying. To optimize all the desirable properties, in thepreferred embodiment one or more additional solvents are added, and theconcentration adjusted, usually by distilling, just prior to coating.Utilizing the boiling point information given above, a solvent with ahigher boiling point may be added to retard the overall drying process,or a solvent with a lower boiling point may be added to speed up thedrying process. N-butyl acetate may be added to increase the solubility.For example, when n-butyl acetate is added as a third solvent, to aprecursor that contains both xylenes and 2-methoxyethanol, preferably ina ratio of approximately 50% xylenes, 20% methoxylethanol, and 30%n-butyl acetate, the resulting thin films have a more even surface andhave fewer cracks, spats, and less precipitation of microparticles.Since methoxyethanol is not a good solvent for metal esters such as2-ethylhexanotes, a recommended solvent for these materials is 50%xylenes and 50% n-butyl acetate. The improvement is especiallysignificant when strontium is one of the metals in the precursor.Generally, solvents with a boiling point above 200° C. are not suitable,even if they have good solubility, because the high evaporationtemperature and low vapor pressure are not compatible with the spin-onand drying processes used in the preferred embodiment of the invention.The addition of ethylene glycol and formamide as evaporation speedcontrol additives has also been found to be effective to control thecracking problem.

In steps 85A and 86A, or steps 85B and 86B, a substrate is provided andprepared for coating. Almost any substrate that will support a thin filmand is compatible with the materials and processes described herein maybe used. Some of these substrates include oxidized or non-oxidizedsilicon or gallium arsenide semiconducting wafers, with or withoutintegrated circuits and/or metalized layers added, plates of silicon orglass, and other electronic device chips. For the exemplary devices ofthis disclosure, the substrates were metalized substrates 18 as shown inFIG. 3. The fabrication of the substrate 18 is described in detail inprior application Ser. No. 07/981,133 referred to above, and will not berepeated herein. While platinum with a titanium adhesion layer is themetalization used in the examples discussed, numerous other metals maybe used such as platinum with an adhesion layer of tantalum, tungsten,molybdenum, chromium, nickel or alloys of these metals, and titaniumnitride. Sputtering or vacuum deposition are the preferred depositionprocesses, though other metalization processes may be used. Heating ofthe substrates during the metalization deposition is effective toincrease adhesion. It has been found that prebaking of the metalizedsubstrate at a temperature that is higher than or equal to thetemperature of any of the subsequent processes performed on the wafer10, which processes are described below, is usually necessary tooptimize the electronic properties of the thin film 30. The prebakingstep 86A comprises baking in an oxygen atmosphere, preferably with anoxygen content of between 500° C. and 1000° C. prior to the coating step87. Preferably the wafer 10 is baked in a diffusion furnace. Thesubstrate prebake step 86A removes water and organic impurities from thesubstrate surface. More importantly, the prebaking decreases theinternal stress of the metal layer 28 through the annealing effect ofthe prebaking and the partial oxidation and interdiffusion of theadhesion layer 26 metal. All this increases the adhesion between thesubstrate 18 and the layered superlattice film 30 and minimizes thepeeling problem. Further, if the adhesion layer 26 is a transitionmetal, the partial oxidation stabilizes the metal chemically. Thereforethe number of mobile atoms penetrating into the layered superlatticelayer 30 through the platinum layer 28 is drastically decreased, and thelayered superlattice layer 30 crystallizes smoothly without defects dueto the diffused ions. If the substrate is not metallized, then thesilicon or other wafer is dehydrated at a lower temperature.

The precursor mixing, distillation, solvent control, and concentrationcontrol steps 81, 82, and 83 have been discussed separately and linearlyfor clarity. However, these steps can be combined and/or ordereddifferently depending on the particular liquids used, whether oneintends to store the precursor or use it immediately, etc. For example,distillation is usually part of solvent concentration control, as wellas being useful for removing unwanted by-products, and thus bothfunctions are often done together. As another example, mixing andsolvent control often share the same physical operation, such as addingparticular reactants and solvents to the precursor solution in apredetermined order. As a third example, any of these steps of mixing,distilling, and solvent and concentration control may be repeatedseveral times during the total process of preparing a precursor.

The mixed, distilled, and adjusted precursor solution is then coated onthe substrate 18. Preferably the coating is done by a spin-on process.The preferred precursor solution concentration ranges from about 0.01 to0.50 M (moles/liter) of the empirical formula having subscriptsnormalized by proportional adjustment to a value of Bi₂). The preferredspin speed ranges from 500 rpm to 5000 rpm.

The spin-on process and the misted deposition process remove some of thesolvent, but some solvent remains after the coating. This solvent isremoved from the wet film in a drying step 88. At the same time, theheating causes thermal decomposition of the organic elements in the thinfilm, which also vaporize and are removed from the thin film. Thisresults in a solid thin film of the layered superlattice material 30 ina precrystallized amorphous state. This dried film is sufficiently rigidto support the next spin-on coat. The drying temperature must be abovethe boiling point of the solvent, and preferably above the thermaldecomposition temperature of the organics in precursor solution. Thepreferred drying temperature is between 150° C. and 400° C. and dependson the specific precursor used. The drying step may comprise a singledrying step at a single temperature, or multiple step drying process atseveral different temperatures, such as a ramping up and down oftemperature. The multiple step drying process is useful to preventcracking and bubbling of the thin film which can occur due to excessivevolume shrinkage by too rapid temperature rise. An electric hot plate ispreferably used to perform the drying step 88.

The drying step 88 is optionally followed by an RTP bake step 89.Radiation from a halogen lamp, and infrared lamp, or an ultraviolet lampprovides the source of heat for the RTP bake step. In the examples, anAG Associates model 410 Heat Pulser utilizing a halogen source was used.Preferably, the RTP bake is performed in an oxygen atmosphere of between20% and 100% oxygen, at a temperature between 500° C. and 850° C., witha ramping rate between 1° C./sec and 200° C./sec, and with a holdingtime of 5 seconds to 300 seconds. Any residual organics are burned outand vaporized during the RTP process. At the same time, the rapidtemperature rise of the RTP bake promotes nucleation, i.e. thegeneration of numerous small crystalline grains of the layeredsuperlattice material in the solid film 30. These grains act as nucleiupon which further crystallization can occur. The presence of oxygen inthe bake process is essential in forming these grains.

The thickness of a single coat, via the spin process or otherwise, isvery important to prevent cracking due to volume shrinkage during thefollowing heating steps 88, 89, and 92. To obtain a crack-free film, asingle spin-coat layer must be less than 2000 Å after the bake step 89.Therefore, multiple coating is necessary to achieve film thicknessesgreater than 2000 Å. The preferred film fabrication process includes RTPbaking for each spin-on coat. That is, as shown in FIG. 1, the substrate18 is coated, dried, and RTP baked, and then the process 90 is repeatedas often as necessary to achieve the desired thickness. However, the RTPbake step is not essential for every coat. One RTP bake step for everytwo coats is practical, and even just one RTP bake step at the end of aseries of coats is strongly effective in improving the electronicproperties of most layered superlattice ferroelectrics. For a limitednumber of specific precursor/layered superlattice material compositions,particularly ones utilizing concentrations of bismuth in excess ofstoichiometry, the RTP bake step 89 is not necessary.

Once the desired film thickness has been obtained, the dried andpreferably baked film is annealed in step 92, which is referred to as afirst anneal to distinguish it from subsequent anneals. The first annealis preferably performed in an oxygen atmosphere in a furnace. The oxygenconcentration is preferably 20% to 100%, and the temperature is abovethe crystallization temperature of the particular layered superlatticematerial 30. Generally, for the materials of the invention, thistemperature is above 700° C. To prevent evaporation of elements from thelayered superlattice material 30 and to prevent thermal damage to thesubstrate, including damage to integrated circuits already in place, theannealing temperature is preferably kept below 850° C. Preferably theannealing for strontium bismuth tantalate is done at about 800° C. for30 to 90 minutes, and is in a similar range for most other layeredsuperlattice materials. Again, the presence of oxygen is important inthis first anneal step. The numerous nuclei, small grains generated bythe RTP bake step, grow, and a well-crystallized ferroelectric film isformed under the oxygen-rich atmosphere.

After the first anneal, the second or top electrode 32 is formed.Preferably the electrode is formed by RF sputtering of a platinum singlelayer, but it also may be formed by DC sputtering, ion beam sputtering,vacuum deposition or other appropriate deposition process. If desirablefor the electronic device design, before the metal deposition, thelayered superlattice material 30 may be patterned using conventionalphotolithography and etching, and the top electrode 32 is then patternedin a second process after deposition. In the examples described herein,the top electrode 32 and layered superlattice material 30 are patternedtogether using conventional photolithography techniques and ion beammilling.

As deposited, the adhesion of the top electrode 32 to the layeredsuperlattice material is usually weak. Preferably, the adhesion isimproved by a heat treatment. The wafer 10 including the layeredsuperlattice film 30 covered by the top electrode 32 may be annealedbefore the patterning step 95 described above in a heat treatmentdesignated in FIG. 1 as the second anneal (1) step 94, after thepatterning step 95 by a heat treatment designated in FIG. 1 as thesecond anneal (2) step 96, or both before and after the patterning step95. The second anneal is preferably performed in an electric furnace ata temperature between 500° C. and the first anneal temperature. A secondanneal below 500° C. does not improve the adhesion of electrode 32, andthe resulting capacitor devices are sometimes extremely leaky, andshorted in the worst cases.

The second anneal releases the internal stress in the top electrode 32and in the interface between the electrode 32 and the layeredsuperlattice material 30. At the same time, the second annealing step94, 96 reconstructs microstructure in the layered superlattice material30 resulting from the sputtering of the top electrode, and as a resultimproves the properties of the material. The effect is the same whetherthe second anneal is performed before or after the patterning step 95.The effect of oxygen ambient during the second anneal is not as clear asit is in the case of RTP bake 89 and the first anneal 92, because thelayered superlattice material 30 is covered by the top electrode and notexposed to the ambient atmosphere. With regard to most electricalproperties, inert gas, such as helium, argon, and nitrogen may be usedwith approximately the same result as with oxygen. However, it has beenfound that an oxygen atmosphere during the second anneal improves thecrystallographic order at the interface of the electrode 32 and layeredsuperlattice material 30 as well as the symmetry of the hysteresiscurve.

3. Examples of the Fabrication Process and Property Dependence

Below, examples of the fabrication process according to the invention asapplied to a wafer 10 as shown in FIGS. 2 and 3 are given. Followingeach of the examples, there is a discussion of the electrical/electronicproperties illustrated in the figures. The figures include hysteresiscurves, such as FIG. 5, and material endurance or "fatigue" curves suchas FIG. 6. The hysteresis curves are given in terms of either theapplied voltage in volts, as for example in FIG. 5, or the electricfield in kilovolts per centimeter, as for example in FIG. 7, versus thepolarization charge in microcoulombs per centimeter squared. Generally,the hysteresis is shown for three different voltages (or fields)generally, 2 volts, 4 volts, and 6 volts. As is well-known, hysteresiscurves which suggest good ferroelectric properties tend to be relativelyboxy and long in the direction of polarization, rather than thin andlinear. The hysteresis measurements were all made on an uncompensatedSawyer-Tower circuit unless otherwise noted. The endurance or "fatigue"curves give the polarization charge, 2Pr, in microcoulombs per squarecentimeter versus the number of cycles. The polarization charge 2Pr isthe charge created by switching a capacitor such as 16 from a statewhere it is fully polarized in one direction, say the upward verticaldirection in FIG. 3, to the opposite fully polarized state, which wouldbe the downward vertical direction in FIG. 3. Here, by "fully polarized"means the state in which the ferroelectric material has been polarizedfully and the field removed. In terms of an hysteresis curve, such asshown in FIG. 5, it is the difference between Pr₊, the point where thehysteresis curve crosses the positive polarization axis (y-axis), andPr₋, the point where the hysteresis curve crosses the negativepolarization axis. Unless otherwise noted, the 2Pr value is taken fromthe hysteresis measurement at the highest voltage. The higher the valueof 2Pr, the better will be the performance of the material inferroelectric memories and other applications. A cycle is defined as thecapacitor, such as 16, being switched through one square pulse. Thispolarization, 2Pr, is approximately twice the remnant polarization, Pr.Other figures, such as FIG. 11, also show the value 2Ec, which is givenin kilovolts per cm, versus some other parameter, such as the amount ofbismuth in the stoichiometry (FIG. 11). The parameter 2Ec is equal tothe sum of the coercive field on the positive side, Ec+, and thecoercive field on the negative side, Ec-, upon a voltage change,generally taken as from -6 to +6 volts for the figures shown. Thecoercive field is a measure of the size of the field that is required toswitch the material from one polarization state to another. For apractical electronic device, it should be high enough that stray fieldswill not cause polarization switching, but if it is too high, largevoltages will be required to operate the device. Other parameters andterms used in the figures and discussion should be clear from thecontext.

EXAMPLE 1 Formation of a Strontium Bismuth Tantalate FerroelectricCapacitor

A wafer 10 including a number of capacitors 12, 24, 16, etc. wasfabricated in which the layered superlattice material 30 was strontiumbismuth tantalate (SrBi₂ Ta₂ O₉). The precursor solution comprisedstrontium 2-ethylhexanoate, bismuth 2-ethylhexanoate, and tantalum2-ethylhexanoate in a xylenes solvent. The plural "xylenes" is usedinstead of the singular "xylene", because commercially available xyleneincludes three different fractionations of xylene. The three metal2-ethylhexanoates were mixed in a proportion such that the strontium andtantalum were present in the mixed precursor in stoichiometricproportions, while the bismuth was present in 110% of stoichiometry. Themolarity of the solution was approximately 0.2 moles per liter. Theprecursor was diluted to 0.13 moles per liter by the addition of n-butylacetate. A substrate 18 comprising a single crystal silicon layer 22, a5000 Å thick layer 24 of silicon dioxide, a 200 Å thick layer 26 oftitanium, and a 2000 Å thick layer 28 of platinum was prebaked at 800°C. in a diffusion furnace for 30 minutes with an oxygen flow of 6liters/min. An eyedropper was used to place 1 ml of the SrBi₂ Ta₂ O₉precursor solution on the substrate 18. The wafer was spun at 1500 RPMfor 40 seconds. The wafer 10 was then placed on a hot plate and baked inair at about 170° C. for 5 minutes and then at 250° C. for another 5minutes. The wafer 10 was then RTP baked at 725° C. with a ramping rateof 125° C./sec, a hold time of 30 seconds, a natural cool time of 6minutes, and an ambient oxygen flow of approximately 100-200 cc/minute.The steps from using an eyedropper to deposit solution on the waferthrough RTP baking were repeated for another coat. The wafer was thentransferred to a diffusion furnace and annealed at 800° C. in an oxygenflow of 6 l/min for 60 minutes. The top layer 32 of 2000 Å platinum wassputtered, a resist was applied, followed by a standard photo maskprocess, an ion mill etch, an IPC strip and a second anneal at 800° C.in an oxygen flow of about 6 l/min for 30 minutes. The final thicknessof the layered superlattice film 30 was 2000 Å.

FIG. 5 shows initial hysteresis curves for the SrBi₂ Ta₂ O₉ samplefabricated in Example 1 measured at FIG. 6 is a graph of 2Pr versusnumber of cycles for the sample of FIG. 5 taken from a series of 20hysteresis curves such as those of FIG. 5; this may be referred to as anendurance or "fatigue" curve as it demonstrates the decline in 2Pr,which may be interpreted as the amount of fatigue of the material, overthe number of cycles switched. FIG. 6, shows that the fatigue over2×10¹⁰ % v amplitude sine wave cycles is less than 10%. Moreover thevalue of 2Pr is about 17 μC/cm 2.

The basic process for making low-fatigue layered superlattice materialsand devices utilizing such materials, as discussed and claimed in thecopending U.S. patent application Ser. No. 07/981,133, results insimilar excellent low-fatigue results as shown in FIGS. 4 and 5. In boththe materials made according to the basic process of the priordisclosure and the materials made with the added improvements of thepresent disclosure, the coercive field is such that electronic devicesthat operate in the range of 2 to 10 volts, the standard range forintegrated circuits, are possible. With the improved process of thepresent disclosure, polarizabilities in the range of 15 μC/cm² to morethan 25 μC/cm² are possible, while keeping the same excellent resistanceto fatigue and excellent coercive field size.

EXAMPLE 2 Control of Electronic Properties by Varying the BismuthContent

The effect of excess bismuth content on the properties of strontiumbismuth tantalate was studied utilizing various bismuth concentrationsin the precursor solutions that yielded corresponding strontium bismuthtantalate layered superlattice material. This variation overcame theproblem of the long annealing time required to reach the maximum 2Pr forthe excess bismuth samples. By controlling the bismuth content of theprecursor, one can control the electronic properties, such as 2Pr, 2Ec,and the resistance to fatigue. This feature of the invention leads tothe ability to design precursor solutions corresponding to optimizedelectronic performance in the resultant metal oxide materials.

A series of ten wafers 10 including a number of capacitors 12, 24, 16,etc. was fabricated in which the layered superlattice material 30 wasstrontium bismuth tantalate (SrBi₂ Ta₂ O₉). The precursor solutioncomprised strontium 2-ethylhexanoate, bismuth 2-ethylhexanoate, andtantalum 2-ethylhexanoate in a xylenes solvent. The three metal2-ethylhexanoates were mixed in a proportion such that the strontium andtantalum were present in the mixed precursor in stoichiometricproportions, while the bismuth was present in the following proportionsdifferent percentage of stoichiometry for each of the ten wafers: 50%;80%; 95%; 100%; 105%; 110%; 120%; 130%; 140%; and 150% of thestoichiometrically required amount of Formula (1) (i.e., Bi₂). Themolarity of the solution was approximately 0.09 moles per liter. Asubstrate 18 comprising a single crystal silicon layer 22, a 5000 Åthick layer 24 of silicon dioxide, a 200 Å thick layer 26 of titanium,and a 2000 Å thick layer 28 of platinum was prebaked at 800° C. in adiffusion furnace for 30 minutes with an oxygen flow of 6 liters/min. Aneyedropper was used to place 1 ml of the SrBi₂ Ta₂ O₉ precursor solutionon the substrate 18. The wafer was spun at 2000 RPM for 40 seconds. Thewafer 10 was then placed on a hot plate and baked in air at about 180°C. for 5 minutes and then at 250° C. for another 5 minutes. The wafer 10was then RTP baked at 725° C. with a ramping rate of 125° C./sec, a holdtime of 30 seconds, a natural cool time of 6 minutes, and an ambientoxygen flow of about 100-200 cc/minute. The steps from using aneyedropper to depositing solution on the wafer through RTP baking wererepeated for another coat. The wafer was then transferred to a diffusionfurnace and annealed at 800° C. in an oxygen flow of 6 l/min for 30minutes. The top layer 32 of 2000 Å platinum was sputtered, a resist wasapplied, followed by a standard photo mask process, an ion mill etch, anIPC strip and a second anneal at 800° C. in an oxygen flow of 6 l/minfor 30 minutes. The final thickness of the layered superlattice film 30was 1900 Å to 2100 Å.

FIG. 7 depicts polarization hysteresis curves for each of the tensamples made according to the process of Example 2, and includes a plotof charge in μC/cm² versus applied voltage for each sample. The Y-axisscale is proportional to a charge range of -20 to 20 μC/cm² for eachhysteresis loop. As indicated above, all samples were prepared with anRTP bake at about 725° C. The best curve separations occurred forbismuth factors ranging from 0.95 to 1.2, or 95% to 120% of thestoichiometric bismuth amount. The best polarizations were obtained fromsamples having bismuth factors ranging from about 1.10 to 1.40.

FIG. 8 depicts values of 2Pr and 2Ec at 6V corresponding to thehysteresis curves of FIG. 7. FIG. 8 shows that the material is clearlyferroelectric above 50% of bismuth stoichiometry. As the amount ofbismuth increases, so does 2Pr and 2Ec. At about 100% of stoichiometry,2Ec peaks and then decreases steadily until it becomes relatively flatat about 130% of stoichiometry. 2Pr peaks at about 120% to 130% ofstoichiometry [α=0.4 in the formula (3) or (4)] and then decreasesgradually. The upper limit of bismuth concentration is defined by theelectrical shorting of the thin film due to the degradation of filmquality caused by excessive grain growth or migration of excess bismuth

FIG. 9 depicts fatigue endurance results for the respective samples ofExample 2. All of the samples show excellent resistance to fatigue.Fatigue resistance, accordingly, does not depend on the bismuth contentas long as the material is ferroelectric. These curves were obtained byrepeqatedly polarizing the respective samples with a sine wave having anamplitude of 5.7V.

The manufacturing and performance improvement principles of films havingexcess bismuth are also applicable to other elements which form highvapor pressure compounds during fabrication of layered superlatticematerials. In addition to bismuth, other such elements include lead,thallium and antimony.

4. Dependence of Electronic Properties on the Elements Comprising theLayered Superlattice Material

In this section various solid solutions of strontium bismuth tantalate,strontium bismuth niobate, strontium bismuth titanate, and strontiumbismuth zirconate shall be discussed. To shorten the technicaldescription and make the figures easier to read, strontium bismuthtantalate will sometimes be referred to as tantalate or abbreviated asTa, strontium bismuth niobate will sometimes be referred to as niobateor abbreviated as Nb, strontium bismuth titanate will sometimes bereferred to as titanate or abbreviated as Ti, and strontium bismuthzirconate will sometimes be referred to as zirconate or abbreviated asZr. This designation should always be clear from the context. Thismethod of designating the layered superlattice materials helpsilluminate the utility of the methods of the invention for designingelectronic devices.

In the following example, layered superlattice materials comprising asolid solution of strontium bismuth tantalate (Ta) and strontium bismuthniobate (Nb) were investigated.

EXAMPLE 3 Strontium Bismuth Tantalum Niobate (TaNb)

A series of wafers 10 including a number of capacitors 12, 24, 16, etc.was fabricated in which the layered superlattice material 30 wasstrontium bismuth tantalum niobate, i.e.,

    SrBi.sub.2 (Ta.sub.y,Nb.sub.1-y).sub.2 O.sub.9,            (17)

wherein y ranges from 0 to 1.

A variety of respective solid solutions of Ta and Nb were fabricatedaccording to Formula (17). The precursor solution comprised strontium2-ethylhexanoate, bismuth 2-ethylhexanoate, tantalum 2-ethylhexanoateand niobium 2-ethylhexanoate in a xylenes solvent. The four metal2-ethylhexanoates were mixed in a proportion such that the strontium,bismuth, tantalum, and niobium were present in the mixed precursor instoichiometric proportions, with y having a progression of values from 0to 1. The molarity of the solution was approximately 0.10 moles perliter. No dilution with a second solvent was performed. A substrate 18comprising a single crystal silicon layer 22, a 5000 Å thick layer 24 ofsilicon dioxide, a 200 Å thick layer 26 of titanium, and a 2000 Å thicklayer 28 of platinum was prebaked at 800° C. in a diffusion furnace for30 minutes with an oxygen flow of about 6 liters/min. An eyedropper wasused to place 1 ml of the [SrBi₂ (Ta_(y),Nb_(1-y))₂ O₉ ] precursorsolution on the substrate 18. The wafer was spun at 1500 RPM for 40seconds. The wafer 10 was then placed on a hot plate and baked in air atabout 180° C. for 5 minutes and then at 250° C. for another 5 minutes.The wafer 10 was then RTP baked at 725° C. with a ramping rate of 125°C./sec, a hold time of 30 seconds, a natural cool time of 6 minutes, andan ambient oxygen flow of 100 to 200 cc/minute. The steps from using aneyedropper to deposit solution on the wafer through RTP baking wererepeated for another coat. The wafer was then transferred to a diffusionfurnace and annealed at 800° C. in an oxygen flow of 6 l/min for 60minutes. The top layer 32 of 2000 Å platinum was sputtered, a resist wasapplied, followed by a standard photo mask process, an ion mill etch, anIPC strip and a second anneal at 800° C. in an oxygen flow of 6 l/minfor 30 minutes. The final thickness of the films 30 ranged between 1400Å and 2100 Å, depending on the sample.

FIG. 10 depicts polarization hysteresis curves for five samples ofstrontium bismuth tantalum niobate (TaNb) having the following y values(Formula (17)) in terms of Ta percentage: 100% (SrBi₂ Ta₂ O₉); 70%; 50%;30%; and 0% (SrBi₂ Nb₂ O₉). The Y-axis scale for each curve extends overa range between -20 and 20 μC/cm². The voltages at which the hysteresiscurves were taken were 2, 4, and 6 volts as before.

FIG. 11 is a plot of 2Pr and 2Ec versus the percentage of Nb, ascompared to Ta, in Formula (17), with 2Pr and 2Ec being measured asbefore from ±6 volt hysteresis curves. The X-axis represents a value of(1-y). The substitution of niobium for the tantalum caused both 2Pr and2Ec to increase dramatically. FIG. 12 shows the fatigue curves out to2×10¹⁰ cycles for five different samples of TaNb with five differentratios of Ta to Nb. The samples were fatigued at 300 kV/cm and thehysteresis curves from which the data for the fatigue curves were takenwere performed at 250 Kv/cm. It is further noted that if the samemolarity is used for precursor solutions, the viscosity of the solutionschanges with the Ta/Nb ratio, with the Ta precursor being more viscousthan the Nb precursor. As a result the final thickness of the 100% Tafilms is approximately twice as thick as the 100% Nb films. Further, Taenriched films tend to saturate their hysteresis at a lower voltage thanNb enriched films. Thus the values of 2Pr in FIG. 19 for the variousratios differ from those in FIG. 11. The resistance to fatigue isexcellent for all samples. However, the large coercive field for sampleswith a high percentage of Nb would result in devices requiring largevoltages to operate. Therefore, in view of practical electronic deviceapplications, an amount of tantalum in SrBi₂ (Ta_(y),Nb_(1-y))₂ O₉ withy between 0.5 and 1.0 is recommended.

In the crystalline structure of strontium bismuth niobate, niobium hasthe same valence and almost the same ionic radii as tantalum instrontium bismuth tantalate. Thus, tantalum and niobium can substitutefor one another in the crystal structure without restriction. All of theresulting materials are excellent ferroelectrics. The next exampleillustrates the result when two layered superlattice materials withsignificantly different crystal structure are mixed in solid solution.

EXAMPLE 4 Strontium Bismuth Tantalum Titanate (TiTa)

A series of wafers 10 including a number of capacitors 12, 24, 16, etc.was fabricated in which the layered superlattice material 30 wasstrontium bismuth tantalum titanate,

    SrBi.sub.4-2y (Ta.sub.y,Ti.sub.2-2y).sub.2 O.sub.15-6y,    (18)

wherein y ranges from 0 to 1. Respective samples containing a solidsolution of Ti and Ta were fabricated with different y values. Theprecursor solution comprised strontium 2-ethylhexanoate, bismuth2-ethylhexanoate, tantalum 2-ethylhexanoate and titanium2-ethylhexanoate in a xylenes solvent. The four metal 2-ethylhexanoateswere mixed in a proportion such that the strontium, bismuth, tantalum,and titanium were present in the mixed precursor in stoichiometricproportions, with y taking on a series of values from 0 to 1. Themolarity of the solution was approximately 0.07 moles per liter. Nodilution with a second solvent was performed. A substrate 18 comprisinga single crystal silicon layer 22, a 5000 Å thick layer 24 of silicondioxide, a 200 Å thick layer 26 of titanium, and a 2000 Å thick layer 28of platinum was prebaked at 800° C. in a diffusion furnace for 30minutes with an oxygen flow of 6 liters/min. An eyedropper was used toplace 1 ml of Formula (18) precursor solution on the substrate 18. Thewafer was spun at 1500 RPM for 40 seconds. The wafer 10 was then placedon a hot plate and baked in air at about 180° C. for 5 minutes and thenat 250° C. for another 5 minutes. The wafer 10 was then RTP baked at725° C. with a ramping rate of 125° C./sec, a hold time of 30 seconds, anatural cool time of 6 minutes, and an ambient oxygen flow of 100-200cc/minute. The steps from using an eyedropper to deposit solution on thewafer through RTP baking were repeated for three coats. The wafer wasthen transferred to a diffusion furnace and annealed at 800° C. in anoxygen flow of 6 l/min for 30 minutes. The top layer 32 of 2000 Åplatinum was sputtered, a resist was applied, followed by a standardphoto mask process, an ion mill etch, an IPC strip and a second annealat 800° C. in an oxygen flow of 6 l/min for 30 minutes. The finalthickness of the layered superlattice films 30 ranged between 2000 Å and3500 Å, depending on the sample.

FIG. 13 depicts polarization hysteresis curves for six different samplesthat were produced according to Formula (18), i.e., y values indicatingthe following percentages of Ti: 100% (SrBi₄ Ti₄ O₁₅); 80%; 50%; 33%;20%; and 0% (SrBi₂ Ta₂ O₉). The voltages at which the hysteresis curveswere run was 2, 4, and 6 volts as before. The Y-axis scale represents arange of from -25 to 25 μC/cm² for each hysteresis curve. In thisinstance, while both the strontium bismuth titanate and the strontiumbismuth tantalate are excellent ferroelectrics, solid solutions of thetwo near 50/50 ratios are not. Moreover a broad range of ferroelectricproperties, such as values of 2Pr and 2Ec are represented near the twoextremes of the solid solutions.

EXAMPLE 5 Strontium Bismuth Niobium Titanate (TiNb)

Another series of wafers 10 including a number of capacitors 12, 24, 16,etc. was fabricated in which the layered superlattice material 30 wasstrontium bismuth niobium titanate

    SrBi.sub.4-2z (Nb.sub.Z,Ti.sub.2-2z).sub.2 O.sub.15-6z,    (19)

wheerein z ranges from 0 to 1. Respective samples were preparedaccording to Formula (19). The precursor comprised strontium2-ethylhexanoate, bismuth 2-ethylhexanoate, niobium 2-ethylhexanoate andtitanium 2-ethylhexanoate in a xylenes solvent. The four metal2-ethylhexanoates were mixed in a proportion such that the strontium,bismuth, niobium and titanium were present in the mixed precursor instoichiometric proportions, with z taking on a series of values from 0to 1. The molarity of the solution was approximately in the range 0.07moles per liter to 0.09 moles per liter depending on the sample.Otherwise the fabrication process was the same as for Example 4 above.The final thickness of the film 30 was between 2200 Å and 2650 Ådepending on the sample.

FIG. 14 depicts polarization hysteresis curves for six different samplesof strontium bismuth niobium titanate (TiNb) having various z-valuepercentages of Ti according to Formula (19): 100% (SrBi₄ Ti₄ O₁₅); 80%;50%; 33%; 20%; and 0% (SrBi₂ Nb₂ O₉). The voltages at which thehysteresis curves were run was 2, 4, and 6 volts as before. The Y-axisscale represents a range of from -25 to 25 μC/cm² for each hysteresiscurve. Again, while both the strontium bismuth titanate and thestrontium bismuth niobate are excellent ferroelectrics, solid solutionsof the two near 50/50 ratios are not. Also a broad range offerroelectric properties, such as values of 2Pr and 2Ec are representednear the two extremes of the solid solutions.

EXAMPLE 6 Strontium Bismuth Tantalum Niobium Titanate (TiTaNb)

Another series of wafers 10 including a number of capacitors 12, 24, 16,etc. was fabricated in which the layered superlattice material 30 wasstrontium bismuth niobium tantalum titanate, i.e.,

    SrBi.sub.4-2x {(Ta.sub.y,Nb.sub.1-y).sub.x,Ti.sub.2-2x }.sub.2 O.sub.15-6x, (20)

wherein x and y range from 0 to 1. That is, a solid solution of Ti, Ta,and Nb, was fabricated. The precursor solution comprised strontium2-ethylhexanoate, bismuth 2-ethylhexanoate, tantalum 2-ethylhexanoate,niobium 2-ethylhexanoate, and titanium 2-ethylhexanoate in a xylenessolvent. The five metal 2-ethylhexanoates were mixed in a proportionsuch that the strontium, bismuth, niobium and titanium were present inthe mixed precursor in stoichiometric proportions, with x and y takingon a series of values from 0 to 1. Otherwise the fabrication process wasthe same as for Example 6 above. The final thickness of the film 30 wasbetween 1850 Å and 24000 Å depending on the sample.

All three of the materials strontium bismuth tantalate (Ta), strontiumbismuth niobate (Nb), and strontium bismuth titanate (Ti) may be mixedin solid solution in arbitrary ratio, making a single mixedferroelectric phase, which can be represented Formula (20).

Polarization hysteresis measurements were conducted on respectivesamples having the following percentages of Ti/Ta/Nb, in terms ofxy/x(1-y)/(2-2x): 100%/0%/0% (SrBi₄ Ti₄ O₁₅); 81%/10%/09%; 52%/25%/23%;35%/34%/31%; 14%/45%/41%; and 0%/50%/50% (SrBi₂ TaNbO₉). The voltages atwhich the hysteresis curves were run was 2, 4, and 6 volts as before.Again, while strontium bismuth titanate and strontium bismuth tantalumniobate are excellent ferroelectrics, solid solutions of the two near50/50 ratios are not. Again, a broad range of ferroelectric properties,such as values of 2Pr and 2Ec are represented near the two extremes ofthe solid solutions.

FIG. 15 is a three dimensional diagram (represented in two-dimensions)showing 2Pr of most of the different layered superlattice materials andsolid solutions fabricated according to Formula (20). Many patternsemerge from this diagram, including the one discussed in relation toFIG. 18, i.e. the rise of 2Pr from 100% Ta to 100% Nb, the generallylower value of 2Pr toward the center of the diagram, and others thatwere not evident from the isolated data, such as the rise in 2Pr alongthe 50% Ti line as it goes from 50% Ta to 50% Nb. Such patterns permitone to use records such as FIG. 15 to design ferroelectric deviceshaving specific, predictable properties.

FIG. 16 is a graph showing the relation between 2Pr and the percentageof Ti in compositions of layered superlattice materials comprising solidsolutions of Ti, Ta, and Nb having specific percentages of Ta and Nb.Such curves depict slices through the three dimensional diagram of FIG.25 parallel to the Ti axis. Such "slices" make it easier to recognizepatterns that provide design direction and advantages.

FIG. 17 is a table in which some of the data discussed above, includingthe sample thickness, is arranged in groups from which patterns emerge.This table illustrates another way of arranging data in records toassist in the design of electronic devices.

FIG. 18 is a graph showing polarization fatigue curves for variousTiTaNb layered superlattice compositions discussed above. The data showsthat compositions with high Ti content (0.0≦x≦0.2), have a large 2Pr butpoor resistance to fatigue beyond 10⁹ cycles. On the other hand,compositions with about equal Ti and Nb content, show a comparablyexcellent resistant to fatigue to the TaNb compounds. This curve thusillustrates still another method of recording data that can causepatterns useful in design of electronic devices to surface.

There exist numerous other layered superlattice materials, and numerousother elements besides tantalum, niobium, and titanium can also beincluded in the layered superlattice solid solutions discussed above, aswell as numerous other solid solutions. These possibilities are toonumerous to discuss fully here. However, to illustrate this, a solidsolution including zirconium will be discussed in the next example.

EXAMPLE 7 Strontium Bismuth Titanium Zirconate (ZrTi)

Another series of wafers 10 including a number of capacitors 12, 24, 16,etc. was fabricated in which the layered superlattice material 30 wasstrontium bismuth titanium zirconate, i.e.,

    SrBi.sub.4 (Ti.sub.z,Zr.sub.1-z).sub.4 O.sub.15            (21)

wherein z ranges from 0 to 1. That is, a solid solution of Zr and Ti,was fabricated. The precursor solution comprised strontium2-ethylhexanoate, bismuth 2-ethylhexanoate, titanium 2-ethylhexanoateand zirconium 2-ethylhexanoate in a xylenes solvent. The four metal2-ethylhexanoates were mixed in a proportion such that the strontium,bismuth, niobium and titanium were present in the mixed precursor instoichiometric proportions, with z taking on a series of values from 0to 1. The molarity of the solution was diluted to approximately 0.07moles per liter by the addition of n-butyl acetate as a second solvent.The RTP bake step 89 (FIG. 1) was performed with a holding temperatureof 750° C. Otherwise the fabrication process was the same as for Example6 above. The final thickness of the film 30 was between 3000 Å and 3500Å depending on the sample.

Zirconium is in the same transition metal column of the periodic tableof the elements as titanium and can be easily substituted for titaniumin a layered superlattice crystal structure in an arbitrary amount.

FIG. 19 depicts polarization hysteresis curves for six different samplesof ZrTi having the following percentages of Zr: 0% (SrBi₄ Ti₄ O₁₅); 20%;40%; 50%; and 60%. The voltages at which the hysteresis curves were runwas 2, 4, and 6 volts as before. The y-axis scale reperesents a rangefrom -15 to 15 μC/cm² for each hysteresis curve. In this case, theferroelectric properties disappear if more than 50% zirconium is added.This does not mean that the material with more than 50% Zr is not alayered superlattice material; it may merely mean that the ferroelectrictransition temperature changes so that the material is no longerferroelectric at room temperature, and/or that the material becomes alayered superlattice dielectric material. FIG. 20 is a graph of 2Pr and2Ec as a function of Zr percentage, while FIG. 21 shows the fatiguecurves for the material with 10% Zr and the material with 20% Zr. Thefigures show that while 2Pr and 2Ec decrease close to linearly with theaddition of Zr, the resistance to fatigue improves with the addition ofZr, at least up to 20%. Thus, Zr also offers opportunities for devicedesign.

The ZrTi compound can be combined with any of the layered superlatticematerials and solid solutions thereof that were discussed above inarbitrary ratios. The electronic properties of such solid solutions forma continuum with the electronic properties discussed above. Thus eachmetal that forms layered superlattice materials literally adds a newdimension to a diagram such as shown in FIG. 15. Other metals that formlayered superlattice materials and thus add such new dimensions include,of course, strontium and bismuth which have been in all the compoundsdiscussed herein, and also calcium, barium, cadmium, lead, hafnium,tungsten, scandium, yttrium, lanthanum, antimony, chromium, andthallium.

6. Optimizing Superlattice Electrical Performance Through StoichiometricProportioning Techniques

It has been discovered that some of these materials produce crystallinestructures having enhanced stable properties as dielectric orferroelectric materials. This benefit can be realized by taking propercare to combine elements of the formula within optimal stoichiometricratios or molar proportions in the precursor solution. The resultantstoichiometrically proportioned final crystals, as well as the methodsof making them, constitute especially preferred embodiments of thepresent invention. Those skilled in the art will understand that a greatvariety of different superlattice materials may be constructed fromprecursor solutions according to the above-described examples, withcrystalline properties being enhanced through the stoichiometricrelative proportioning of lattice elements, or the addition of smallamounts of metallic oxide dopants.

The precursors may be mixed to include a dopant fraction having ametallic alkoxocarboxylate, alkoxide, or carboxylate, such as tantalum2-ethylhexanoate, to introduce or alter a proportional metallic moietydistribution in the final superlattice materials. For example, aprecursor having sufficient metallic oxygenated organic complexes toprovide a final dielectric crystalline material of the formulation SrBi₂Ta₂ O₉ can be doped to provide a 10% Ta₂ O₅ ("TaO") concentration bymixing sufficient tantalum 2-ethylhexanoate to provide an excess of 10mol % Ta, or Ta₂.2, in the final formula (SrBi₂.0 Ta₂.2 O₉.5).

EXAMPLE 8 Strontium Content Optimization Study

The precursor solution from Example 1 was selected as a primaryprecursor solution for use in further investigations comparing theeffects of different strontium concentrations upon ferroelectric anddielectric properties. The primary precursor solution selected forfurther investigation was one having metallic 2-ethylhexanoates inrelative proportions sufficient to produce a final material having theaverage formula of Sr₁ Bi₂ Ta₂ O₉. The sensitivity analysis wasconducted to determine the effect of strontium content upon theferroelectric properties of field intensity and polarizability for thesuperlattice materials that derive from these precursors. In thissensitivity study, the strontium of the precursor solution was variedaccording to the sensitivity formula

    Sr.sub.w Bi.sub.2.0 Ta.sub.2 O.sub.8+w,                    (22)

wherein the quantity (8+w) represents an oxygen portion derived fromexcess oxygen in the precursor and annealing environments, and providesan oxygen anion charge in an amount sufficient to balance thecorresponding charge from the portion of Sr⁺² cation that varies as w.

In addition to the primary precursor liquid, a series of secondaryprecursor liquids were produced having various strontium concentrationsaccording to the sensitivity formula. These respective precursor liquidswere produced according to the methods described in previous examples,by mixing respective metallic carboxylate fractions in proportionscorresponding to the proportions of metallic elements in the formula. Inthis manner, several secondary precursor liquids were produced havingstrontium molar portions of 0.0, 0.15, 0.25, 0.35, 0.5, 0.6, 0.8, 0.9,1.0, 1.1, 1.2, and 1.4.

A similar set of solutions was produced according to a secondsensitivity formula

    Sr.sub.w Bi.sub.2.2 Ta.sub.2 O.sub.8.3+w,                  (23)

with the strontium content being varied to produce precursor liquidshaving strontium molar proportions of 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1,and 1.2. This excess amount of bismuth was added to components from thevolatilization of bismuth oxide at the annealing temperatures.

Each these precursors was used to manufacture, under identicalconditions, a thin-film wafer having numerous capacitors according tothe method of Example 1. Ferroelectric properties, namely, polarization(as 2Pr) and electric field (as Ec) were measured according toconventional techniques. For example, 2Pr was determined from hysteresismeasurements (charge versus applied voltage) that were conductedaccording to standard protocols on a circuit including a Hewlet Packard3314A function generator and a Hewlet Packard 54502A digitizingoscilloscope.

FIG. 22 depicts the results of ferroelectric property measurements as aplot of polarization and field intensity values for measurementsconducted on the various samples. As indicated by the legend on FIG. 22,the respective lines corresponding to the open and filled circlesrepresent measurements taken on virgin samples of the Formula (22) classof formula variants. The respective lines corresponding to the open andfilled triangles represent second measurements taken on the same samplesunder identical conditions. The respective lines corresponding to theopen and filled squares represent measurements conducted on the virginFormula (23) variants. These results indicate that 2Pr approaches 0 inthe samples having a Sr concentration exceeding 100% of the 1.0 molarportion.

FIG. 23 depicts the individual first-run hysteresis curves for each ofthe dielectric materials according to Formula (22). The respectivecurves represent materials having their formulations varied as indicatedto investigate the hysteresis sensitivity of strontium concentration.These measurements were used to construct the circle curves of FIG. 22.

FIG. 24 depicts the individual second-run hysteresis curves for each ofthe dielectric materials according to Formula (22). The respectivecurves represent materials having their formulations varied toinvestigate the sensitivity of strontium concentration as it affects thehysteresis curve. These measurements were used to construct the trianglecurves of FIG. 22.

FIG. 25 depicts the individual hysteresis curves for each of thedielectric materials according to Formula (23) (10% excess bismuth). Thematerials have their formulations varied to investigate the sensitivityof strontium concentration as it affects the hysteresis curve. Thesemeasurements were used to construct the square curves of FIG. 22.

The results of FIGS. 22 through 25 demonstrate that crystals having poorstability, as indicated by a lack of measurement repeatability andincidence of capacitor shorting, are produced when the Sr concentrationfalls below a 0.35 portion or above a 1.1 portion. In the case ofmaterials formed from precursors having 10% excess bismuth, shortingoccurred for strontium portions above 1.0. For the formulations studied,materials having a Sr concentration falling outside the Sr₀.35 to Sr₁.1range are typically characterized by poor dielectric performance andpoor ferroelectric performance. In contrast, capacitors havingdielectric layers including strontium in a molar portion from about 0.35to about 1.1 are polarizable, exhibit good dielectric characteristics,and have a relatively stable crystalline structure under bias voltagestresses.

FIGS. 26-31 each depict a plurality of hysteresis curves taken fromdifferent strontium bismuth tantalate capacitor sites formed on a singlewafer. The parenthetical numbers, e.g., (8,3), serve to identify acapacitor site on the wafer grid. Each figure corresponds to a differentwafer, with the strontium content changing for the different wafers asindicated on the figures, e.g., FIG. 26 depicts measurements taken fromfour capacitors on a wafer having the formulation Sr₀.8 Bi₂.0 Ta₂.0O₈.8. The precursors used to form these materials had no excess bismuthadded. Among all of these figures, the strontium portion was variedbetween 0.8 and 1.4. At each concentration, the respective curves aresubstantially identical, and exhibit good ferroelectric properties,until, at a strontium portion of about 1.1, the material begins todemonstrate poor ferroelectric properties. The ferroelectric phenomenoncompletely fails for materials having strontium portions of 1.2 andgreater.

FIG. 32 depicts results for polarization fatigue measurements conductedon ferroelectric materials after switching them up to 10¹⁰ cycles. Thesemeasurements were conducted by repeatedly charging and discharging thesamples on a 5.7V sine wave, and measuring the retained charge underidentical conditions each time. The six samples had Sr molar portionsranging from about 0.5 to about 1.1 according to formula (22), and wereformed from precursor solutions having no excess bismuth. Thesematerials exhibited extremely low or negligible levels of fatigue.

EXAMPLE 9 Preventing Shorts in Strontium Bismuth Tantalate Materialsthrough the Addition of A-Site and B-Site Dopants

FIG. 33 depicts hysteresis curves taken from different capacitorsmanufactured on a wafer incorporating a dielectric or ferroelectriclayer having the formulation SrBi₂ Ta₂ O₉ with an additional 10% excessbismuth added to the precursor solution. The metal layers of thesecapacitors are shorted and, therefore, useless as ferroelectriccapacitors.

In contrast, FIG. 34 depicts first run hysteresis results for capacitorsincluding a ferroelectric material made from a precursor having the sameformulation as that which produced the FIG. 33 material, but adding adopant of 10 mol % Bi₂ Ta₂ O₈ to the resultant crystalline material.These dopants are prepared by adding to the stock precursor solution acombination of bismuth 2-ethylhexanoate and tantalum 2-ethylhexanoate.The FIG. 34 capacitors are not shorted, and exhibit good ferroelectricproperties in terms of boxy, rectangular curves.

FIG. 35 depicts second run hysteresis results for the capacitors of FIG.34. The FIG. 35 measurements demonstrated good repeatability betweensuccessive runs.

FIG. 36 depicts hysteresis results for capacitors having ferroelectricmaterials of the SrBi₂ Ta₂ O₉ plus 10% excess bismuth formulation, withan additional 20 mol % TaO_(5/2) dopant added. Again, the TaO_(5/2)dopant alleviated the shorting problem that was observed in FIG. 33.

FIG. 37 depicts several hysteresis curves for metal oxide materials thatwere produced from a based stock of precursor solution corresponding toa SrBi₂ Ta₂ O₉ formula to which 10% excess BiO_(3/2) was added inaddition to a dopant. The respective curves are labeled with the dopantconcentrations, namely, no dopant, 10% Bi₂ Ta₂ O₈, 20% BiO_(3/2), and20% TaO_(5/2). Again, the 10% Bi₂ Ta₂ O₈ and 20% TaO_(5/2) dopants(Curves C and E) remedied the shorting problem that was observed in FIG.33. The 20% BiO_(3/2) dopant failed to remedy the shorting problem.Curve A actually produced a viable ferroelectric material in the absenceof a dopant.

In this manner, the observance of certain stoichiometric proportionswithin the superlattice formulation will result in ferroelectric anddielectric materials having especially stable crystalline structures. Inparticular, these formulations have the formula:

    Sr.sub.w [Bi.sub.4-2x+α {(Ta.sub.y,Nb.sub.1-y).sub.x,(Ti.sub.z,Zr.sub.1-z).sub.2-2x }.sub.2 O.sub.15-6x ].sub.c,                                      (24)

wherein the formula elements are as described above 0≦w≦1, and c≧1. Inthis formula, the especially stable crystals will result when quantityc÷w is ≧1. This ratio may be maintained following a number of especiallypreferred modes. A first mode exists wherein w≦1, and c=1. A second modeexists where w=1 and c≧1. A third mode exists where 0.6≦w≦1.0 and c≧1.

By way of example, with reference to the formula immediately above

    Sr.sub.w [Bi.sub.4-2x+α {(Ta.sub.y,Nb.sub.1-y).sub.x,(Ti.sub.z,Zr.sub.1-z).sub.2-2x }.sub.2 O.sub.15-6x ].sub.c,                                      (25)

as compared to Sr₁.0 Bi₂.0 Ta₂.0 O₉.0 of this example, w is 1.0, x is1.0, α is zero, c is 1, and the quantity (w÷c) is 1.0.

In another formula

    Sr.sub.w (Bi.sub.α Ta.sub.2).sub.y O.sub.z,          (26)

as compared to Sr₁.0 Bi₂.0 Ta₂.0 O₉.0 of this example, w is 1.0, α is2.0, y is 1.0, z is 9.0, and the quantity (w÷y) is 1.0.

In yet another formula

    (SrBi.sub.α).sub.x Ta.sub.y O.sub.z,                 (27)

as compared to Sr₁.0 Bi₂.0 Ta₂.0 O₉.0 of this example, α is 2.0, x is 1,Y is 2, and z is 9.0.

There have been described optimized processes and compositions formaking electronic devices utilizing layered superlattice materials, thedependence of the electronic properties on the processes andcompositions has been demonstrated, and methods for utilizing the dataon electronic properties as a function of layered superlattice processesand compositions to make electronic devices has been described. Itshould be understood that the particular embodiments shown in thedrawings and described within this specification are for purposes ofexample and should not be construed to limit the invention which will bedescribed in the claims below. Further, it is evident that those skilledin the art may now make numerous uses and modifications of the specificembodiment described, without departing from the inventive concepts. Forexample, now that prebaking of the substrate, RTP bake, and bismuthcontent have been identified as critical for layered superlatticematerial optimization, these processes can be combined with conventionalprocesses to provide variations on the processes described. It is alsoevident that the steps recited may in some instances be performed in adifferent order. Or equivalent structures and process may be substitutedfor the various structures and processes described. Or a variety ofdifferent dimensions and materials may be used. Further, now that theimpact of the regularity of the properties of layered superlatticematerials and the variety of the materials on electronic design andmanufacturing has been pointed out, many design and manufacturingprocesses utilizing the concepts disclosed may be devised. Consequently,the invention is to be construed as embracing each and every novelfeature and novel combination of features present in and/or possessed bythe fabrication processes, electronic devices, and electronic devicemanufacturing methods described.

We claim:
 1. A ferroelectric device comprising:a pair of electrodes; alayered superlattice material compound interposed between saidelectrodes and having an average empirical formula

    Sr.sub.w [Bi.sub.4-2x+α {(Ta.sub.y,Nb.sub.1-y).sub.x,(Ti.sub.z,Zr.sub.1-z).sub.2-2x }.sub.2 O.sub.15-6x ].sub.c,

wherein w, c, x, y, z, and α represent numbers corresponding to molarportions of formula substituents, 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0,(x-2)≦α≦1.6(2-x), 0≦w≦1.0, c≦1.0, and (c÷w)≧1.
 2. A ferroelectric deviceas in claim 1, wherein w≦1.
 3. A ferroelectric device as in claim 1,wherein 0.6≦w≦1.0.
 4. A ferroelectric device as in claim 1, wherein thelayered superlattice material compound includes strontium bismuthtantalate and a dopant selected from the group consisting of calcium,barium, cadmium, lead, hafnium, tungsten, scandium, yttrium, lanthanum,antimony, chromium, and thallium.
 5. A ferroelectric device as in claim4, wherein said dopant includes a B-site metal selected from the groupconsisting of hafnium and tungsten.
 6. A ferroelectric devicecomprising:a pair of electrodes; a layered superlattice materialcompound interposed between said electrodes and having an averageempirical formula

    Sr.sub.w (Bi.sub.α Ta.sub.2).sub.y O.sub.z,

wherein w, α, y, and z represent numbers corresponding to molar portionsof formula substituents, 0≦w≦1.0, 0≦y≦1.0, 2≦α≦2.2, 0.6<(w÷y)<1.0, and zis sufficient to balance a charge of said formula by providingsufficient oxygen anion to balance a positive charge from the Sr, Bi,and Ta cations.
 7. A ferroelectric device as in claim 6, wherein saidlayered superlattice material compound includes strontium bismuthtantalate and a dopant selected from the group consisting of calcium,barium, cadmium, lead, titanium, hafnium, tungsten, niobium, zirconium,scandium, yttrium, lanthanum, antimony, chromium, and thallium.
 8. Aferroelectric device as in claim 7, wherein said dopant includes aB-site metal selected from the group consisting of hafnium and tungsten.9. A ferroelectric device comprising:a pair of electrodes; a layeredsuperlattice material compound interposed between said electrodes andhaving an average empirical formula

    (SrBi.sub.α).sub.w Ta.sub.y O.sub.z,

wherein w, y, α and z represent numbers corresponding to molar portionsof formula substituents, 0≦w≦1.0, 0≦y≦1.0, 2≦α≦2.2, 2<y/w, and z issufficient to balance a charge of said formula by providing sufficientoxygen anion to balance a positive charge from the Sr, Bi, and Tacations.
 10. A ferroelectric device as in claim 9, wherein said layeredsuperlattice material compound includes strontium bismuth tantalate anda dopant selected from the group consisting of calcium, barium, cadmium,lead, titanium, hafnium, tungsten, niobium, zirconium, scandium,yttrium, lanthanum, antimony, chromium, and thallium.
 11. Aferroelectric device as in claim 10, wherein said dopant includes aB-site metal selected from the group consisting of hafnium and tungsten.